Communication systems take many forms. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the message signal into a form suitable for transmission over the channel. This processing of the message signal is referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to process the received signal so as to produce an estimate of the original message signal. This processing of the received signal is referred to as demodulation.
Analog and digital transmission methods are used to transmit a message signal over a communication channel. The use of digital methods offers several operational advantages over analog methods, including but not limited to: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of message signals, improved security of communication through the use of encryption and increased capacity.
To transmit a message signal (either analog or digital) over a communication channel having an assigned channel bandwidth, the message signal must be manipulated into a form suitable for efficient transmission over the channel. Modification of the message signal is achieved by means of a process termed modulation. This process involves varying some parameter of a carrier wave in accordance with the message signal in such a way that the spectrum of the modulated wave matches the assigned channel bandwidth. Parameters of a carrier wave that can be varied include amplitude, frequency, and or phase. Correspondingly, the receiver is required to re-create the original message signal from a degraded version of the transmitted signal after propagation through the channel. The re-creation is accomplished by using a process known as demodulation, which is the inverse of the modulation process used in the transmitter.
A spread spectrum system provides, among other things, robustness to jamming, good interference and multipath rejection, and inherently secure communications from eavesdroppers. In a spread spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth of the information itself. However, in a spread spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal (commonly known as a spreading code).
Thus, a spread spectrum system must have two properties: In (1) the transmitted bandwidth should be much greater than the bandwidth or rate of the information being sent and (2) some function other than the information being sent is employed to determine the resulting modulated channel bandwidth.
The essence of the spread spectrum communication involves expanding the bandwidth of a signal, transmitting the expanded signal and recovering the desired signal by remapping the received spread spectrum into the original information bandwidth. Furthermore, in the process of carrying out this series of bandwidth trades, the purpose of spread spectrum techniques is to allow the system to deliver reliable information in a noisy signal environment.
With digital communication, user information such as speech is encoded into sequences of binary information symbols. This encoding is convenient for modulation and is easily error-correction coded for transmission over a potentially degrading communication channel. Such binary information is particularly amenable to transmission using "direct sequence" spread spectrum modulation. With direct sequence, digital information is spread with a spreading code whose bit rate is much higher than the information signal itself. Although the spreading can be accomplished by several methods, the most common is to modulo-2 add each bit of information (generally after appropriate error correction coding) to a sequence of bits of the spreading code. Thus as desired for the spreading process, many bits are generated for each coded information bit that is desired to be transmitted.
Advantages from direct sequence spread spectrum communication systems are obtained since the receiver is knowledgeable of the spreading code used to spread the user signal. As is well known in the art the receiver, after appropriate synchronization to the receive signal, is able to decode the wide bandwidth spread signal using a replica of the spreading sequence. Another advantage of spread spectrum communication systems is the ability to provide multiple access capability. Specifically, cellular telephone communication systems have been designed to incorporate the characteristic of communicating with many remote units on the same communication channel.
One type of multiple access spread spectrum communication system realized with direct sequence spread spectrum is a code division multiple access (CDMA) communication system. In a CDMA communication system, communication between two communication units is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code. As a result, transmitted signals are in the same frequency band of the communication channel and are distinguished only by being assigned unique user spreading codes. Particular transmitted signals are retrieved from the communication channel by despreading a signal which is the sum of signals in the communication channel with a user spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. Specially suited spreading codes may be employed to reduce the interference created by the sum of all the other signals present on the same channel. Orthogonal codes are typically used for this purpose, and of these, the Walsh codes are most common.
One type of spread spectrum transmitter, such as that disclosed in the U.S. Pat. No. 5,515,396 assigned to instant assignee and hereby incorporated by reference, discloses a type of bi-orthogonal multi-rate transmitter that transmits only one spreading code at a time but selects from a plurality of orthogonal spreading codes to transmit data at a higher rate than conventional transmitters. With a fixed bandwidth spectrum however such a system can use up too many codes so that there are not enough spreading codes to be assigned to a multitude of users, such as where several users each use eight spreading codes. An exponentially increasing number of channels must be dedicated to provide for increasing multiples of a basic transmission rate. With a larger number of orthogonal waveforms, receiver complexity becomes restrictive. For example, the bi-orthogonal technique which makes a selection of a single spreading code to be used with a number of information bits equal to the rate multiplier minus one plus one bit to determine the sign or polarity of the selected code, uses up spreading codes exponentially as the rate multiplier increases. For example, for a basic rate of R, and a multiple rate of MR, 2.sup.M-l codes are required (where M is the rate multiplier). Although such bi-orthogonal multi-rate coding schemes offer an advantage by having the peak to average power ratio (in decibels) effectively zero since only one code is transmitted at a time, the increase in traffic demands for many communication systems requires a system that can offer a more efficient use of a limited number of spreading codes.
Another type of spread spectrum multi-rate coding technique, sometimes referred to as the "sum of codes technique", allows for generation of two times a standard rate by simultaneously transmitting the linear sum of two spreading codes which allows multi-rate transmission. This technique typically makes better use of spreading codes than the bi-orthogonal technique. However higher and more variable power amplifier peak to average levels are required thereby increasing the cost and complexity of both the transmitter and associated receiver. Generally, the sum of codes technique increments the rate by adding a new spreading code for each integer multiple of a given rate. A disadvantage of this approach is that the peak to average power ratio of a transmitted signal worsens with each code added.
Consequently there exists a need for a multi-rate communication system for a spread spectrum communication system that substantially overcomes the above drawbacks while maintaining or improving performance comparable with such alternative approaches.